A terrain database is a set of geographic data that includes information about the terrain at certain locations. For instance, the terrain database can provide the latitude, longitude, elevation from sea level, and slope information for locations in a geographic region. In addition to terrain data, the database may also have obstacle data for such things as buildings, radio towers, etc. Terrain databases are utilized in combination with a number of sensors, such as global positioning system (GPS) sensors, to generate functions of actual surface terrain features of a traveling vehicle, such as an aircraft, boat, or automobile. These functions are typically utilized in aircraft to enhance orientation, situational awareness, and guidance, and to provide alerts and warnings to pilots in flight management systems, such as a terrain awareness and warning system (TAWS), enhanced ground proximity warning system (EGPWS), and runway approach systems.
The geographic data in these databases are generated by a variety of methods, including actual physical measurements or remotely generated estimations of the elevations of geographic features in a given area. Generally, the methods using physical measurements take a sample of physical measurements at specific or randomly generated locations and estimate elevations for the remainder of the area not actually measured. To create remote estimations, a remote sensing system approximates an elevation for locations in the geographic region, such as by using light detecting and ranging (LIDAR) mapping, satellite-based mapping, aerial photography, and other known techniques. An example remote sensing system is the interferometric synthetic aperture radar (IFSAR) system used by the Shuttle Radar Topography Mission (SRTM) to acquire topographic data on 80% of the Earth's land mass. Each source data point in these databases represents a specific elevation at a location on the earth's surface (geo-referenced), often correlated to an earth model known as the WGS 84 reference system.
Depending on the method used to generate the terrain database, inaccuracies are introduced into the data. For example, the SRTM data is not as accurate when measuring the elevations of a sandy area, such as a desert, because the signal used in the IFSAR system can penetrate sand. Thus, the actual elevation of the sandy area is often higher than the estimation generated by the SRTM. Besides the inaccuracies inherent in the measurement system used, the method of compiling the data can contribute additional inaccuracies. For instance, a database may be generated from a few fairly precise location data points. However, the information is only accurate at certain locations, and the elevations between these points may be incorrect. For these reasons, databases generated by different source organizations, which use different extrapolation or estimation methodologies, generally provide conflicting information for the same geographic regions.
To compensate for the inaccuracies and ensure the safety of aircraft that must fly over the terrain, a minimum altitude safety envelope is established for the geographic region. Generally, a safety organization, such as the Federal Aviation Administration (FAA), provides a safety envelope for the entire region that is based on a known highest point in the region, often taken from a pre-existing paper map. For example, if the highest mountain in a 50-by-50 mile region has an elevation of 14,556 feet, then the minimum elevation for the entire 50-by-50 mile region will be higher than 14,556 feet. This type of minimum altitude envelope tends to produce many unnecessary alerts and warnings (nuisance alerts), which a pilot may learn to ignore, thus causing the pilot to become desensitized to more important warnings. In addition, a standard method is to apply a fixed buffer to the entire database. These buffers are usually between 1,000 ft and 3,000 ft high and also tend to generate numerous nuisance alerts. It is with respect to these and other considerations that the present invention was developed.